Well, I'm not proposing he get most of the rages (well, for you). Indeed, I'm proposing he gets what, less than 15 minutes of game time? One rage with a fatal status, one rage with a 108+ power move (any level 3 spell, Holy, Giga Volt, Flare, Aero, Flare Star, Shock, etc.)... I dunno, as you pointed out he probably doesn't even need much as far as elements go as you noted (on top of what you listed, he has a floating rage and naturally spoils fire/ice via equips) and he his third way of winning fights, stalling, requires a very specific rage so is out. Maybe one of the "immunes most statuses" rages but I'd have to see how many of those there are.
Regardless, this is an incredibly paltry amount of effort by DL standards for legal skills. Certainly much less time than it takes to get all of Strago's lores, some of which are extreme FAQ-bait ('sup Big Guard, you're actually unlikely to get that one without the Rage, since the other way is Sketch).
The Veldt doesn't give Exp (nor do many other sidequesty things in RPGs), but the Veldt does give AP and gold. It's also the best way to get a number of Lores, and a requirement for getting the ultimate armours of Gau/Relm/Strago/Mog (need to refight SrBehemoth, who fortunately shows up 1-2 times per Veldt Cycle). But even if there were no further rewards, it is a very small amount of time to get something which is unique to a PC. That stuff's legal, unless you want to start punting things which take considerably more time to get (Seraphic Radiance in both games, for an easy example), and in many cases are considerably more FAQ-bait, may be blocked off by points of no return, and/or are vastly inefficient and out-of-the-way for a playthrough to pick up.
Also, as a purely in-game thing, while slapping on an esper to learn an ID spell is faster than hunting down an ID rage on the Veldt (though it's not all that far apart!), this is certainly not the case earlygame. Gau's the only person with ID from the time you get him until you finish the Vector arc (aside from a select few enemies who will ID each other when confused), which is kinda cool against a few enemies. This argument could be applied to other skills too (in some cases with a lot more force behind it, such as for Edgar's Air Anchor, which you get long after you have many other ways to inflict ID, without giving the target a turn first).
